1. Field of the Invention
This invention relates to decoding in a spread spectrum, code division, multiple access (CDMA) communication system and, more particularly, to an apparatus and method of determining a data rate of an encoded user communication for purposes of controlling decoding of the user communication using a viterbi decoder.
2. Related Art
The CDMA system is interference limited, i.e. signals from one user create interference to the other users. Performance of the system depends on the total interference level. The system is designed so that every user transmits as little power as possible and only when it is necessary, thereby allowing the whole system to support more users.
In the IS-95 standard, the data rate depends on the speech activity and can change from frame to frame. However, the receiver does not know the data rates of the received frames. Referring to FIG. 1, a prior art CDMA receiver is shown. The user communication signal is received by a RAKE receiver and passed to three averaging circuits 14a, 14b, and 14c connected in parallel where an average of pairs, 4 tuples, and 8 tuples, respectively, is made. The output of the RAKE receiver is also supplied to the input of a first viterbi decoder 16a. The output of the averaging circuit 14a is supplied to the input of a viterbi decoder 16b. The output of the averaging circuit 14b is supplied to the input of a viterbi decoder 16c. The output of the averaging circuit 14c is supplied to the input of a viterbi decoder 16d. One output from each of the viterbi decoders 16a, 16b, 16c, and 16d is supplied to a selector circuit 24. Another output from each of the viterbi decoders 16a, 16b, 16c, and 16d is supplied through a separate encoder 18a, 18b, 18c and 18d and a separate bit error counter 20a, 20b, 20c and 20d, respectively, to a comparator 22. The comparator 22 outputs a signal to the selector 24 indicative of the which bit error counter has the least bit errors and the selector 24 then selects the output of the corresponding viterbi decoder 16a, 16b, 16c, and 16d as the output.
Therefore, conventionally, in order to decode the received frame correctly, the received frame has to be decoded four times using viterbi decoding for the four data rates 1.2 kbps, 2.4 kbps, 4.8 kbps and 9.6 kbps in rate set 1 or 1.8 kbps, 3.6 kbps, 7.2 kbps and 14.4 kbps in rate set 2. Then the four decoded frames are re-encoded and the re-encoded frames are compared with the original received frame. The embedded CRC bits or the number of bit corrections will then tell which sub-rate was transmitted. However, this method requires high complexity and power consumption because the receiver has to decode the received frame four times.
Various attempts to avoid this problem have been proposed. Channel decoding power consumption at the mobile station can be reduced by determining the sub-rate ("rate determination") prior to applying the viterbi decoder. For such methods, the viterbi decoder is applied once per frame. Papers which describe these methods are: E. Cohen and H. Lou, "Multi-rate Detection for the IS-95 CDMA forward traffic channels", IEEE Global Telecommunications Conference, November 1995 (hereinafter referred to as the "Cohen-Lou method"); E. Cohen and H. Lou, "Multi-rate Detection for the IS-95A CDMA Forward Traffic Channels Using The 13 kbps Speech Coder", IEEE International Conference on Communications, 1996; H. Kwon, S. Ratanamahatana, and J. Shim, "One Viterbi Decoder with Data Rate Estimation for IS-95 CDMA Wireless Communications", IEEE, 1997. Patents which describe such processes are U.S. Pat. Nos. 5,509,020 and 5,796,757. However, such prior art methods are often complex, requiring many arithmetic operations which require more power and are difficult to implement in actual practice with acceptable accuracy.